1. Field of the Invention
The invention relates to a mechanism for cutting single thickness paper and like sheet material and is of the kind having a pivoted blade which cooperates during the cutting operation with a fixed blade. The invention further relates to sheet feeding apparatus, such as may be incorporated, for example, in printers and copiers, where such a mechanism is included for cutting individual sheets of paper from continuous stock.
2. Description of the Prior Art
The requirements for successful cutting of paper or like materials are well understood, and can conveniently be explained using the blade design of the familiar domestic scissor as an example. During the following brief explanation, reference will be made to FIGS. 1 to 4 of the accompanying drawings which illustrate the various desirable characteristics of scissor blades.
The action of cutting sheet material by shearing is illustrated in FIG. 1 where two sharp blades 1 and 2 are shown applying a transverse shear stress to the material 3. In order to minimize the force required to sever the material, scissor blades 4 and 5, shown in FIG. 2, are curved along their lengths so that cutting is progressive from point to point along the line of cut as the blades close. Preferably, the curvature of the blades are such that they present a constant nip angle (.alpha.) to the material along the entire blade length. It is also a requirement that the cutting edges of the blades touch each other only at the point of shearing the material. This involves a further curve along the length of each blade 4 and 5 as shown in FIG. 3 and also relief of the facing surfaces of the blades as shown in FIG. 4.
The manufacture of blades with such complex forms in hardened steel is still largely a hand craft and consequently is an expensive operation.
GB No. 2,051,652A describes a guillotine of the type commonly used in offices for trimming paper in which a pivoted cutting blade cooperates with a fixed cutting blade. In this particular construction of guillotine, the cutting portion of the pivoted blade is formed from substantially rigid sheet material. During the cutting operation, the rigid blade is supported by being sandwiched between the fixed blade member and a rigid backing member to which the sheet material is attached. By this means, the cutting edge of the pivoted blade is restrained from moving laterally away from the fixed blade during cutting operations.
To improve the cutting action of this guillotine, the side face of the rigid backing member may be slightly concavely curved or alternatively the cutting blade may have its ends fixed to the backing member so that the intermediate portion of the blade is flexed outward to some extent away from the backing member toward the fixed blade. Additional restraining means comes into operation during the act of cutting positively to prevent the cutting edges from moving sideways out of contact with each other.
This guillotine has a composite blade construction which is claimed to be less costly than the more usual guillotine in which both blades are formed from heavy gauge tempered steel plate. However, the guillotine is intended not only for trimming single sheets of material, but also for cutting through a stack of sheets and is therefore rugged in construction and heavy in design.